The invention relates to a method of preparing stable slurries for use in coating color formulations that are applied to a base stock in the manufacture of paper or paperboard materials. The slurries include one or more alumina pigments,a nonionic wetting agent or nonionic polymer and a cationic interfacial modifier. The coating colors formed using such slurries may be applied to a paper or paperboard stock during a manufacturing process and dried on the surface thereof to form a smooth, printable surface with good porosity, such as is needed for subsequent printing applications. In comparison to conventional pigment dispersions, the slurries may be prepared in advance and stably maintained under ambient conditions for relatively long periods without gelling, flocculation or other deterioration. In this regard, a high solids content of greater than 70% weight may be achieved while maintaining the viscosity at a level that permits the slurry to be easily poured.
Coating formulations or xe2x80x9ccolorsxe2x80x9d used in the manufacture of printable paper or paperboard products are usually applied to at least one side of a base stock to improve surface smoothness and to provide uniform ink reception and other generally acceptable printing properties. Typically, such formulations include pigments, binders and other additives such as dispersants, viscosity modifiers, lubricants, and chemicals that modify foaming tendencies, microbial susceptibility, pH or physical characteristics such as color. Porous pigments, for example alumina, are important components of such coating formulations because they contribute to surface porosity of the paper, which, in turn, aids ink drainage. The pigments also reduce surface irregularities to provide a smoother product surface. In this regard, a higher proportion of porous pigment produces a corresponding improvement in the smoothness, porosity and ink-receptivity of the coating. These characteristics are particularly desirable for certain end-use applications of the coated paper product, such as inkjet printing. A commonly observed drawback, however, is that the particulate nature of the pigment limits the amount that can effectively be included in the coating formulation during the manufacturing process.
The pigment is usually added during makedown of the coating formulation as a dispersion or slurry of finely divided particles, as this mode of addition is more convenient for bulk manufacturing. The use of such dispersions is, however, limited by the amount of pigment that is actually suspended in the dispersing medium at the time that it is added to the coating formulation. Because the pigments are insoluble, the pigment particles will settle out, resulting in a reduced amount of pigment per unit volume in the dispersion and the coating color. As a consequence, it is difficult to control the total solids content in the dispersion itself and in the final coating formulation. Another disadvantage is that the particles may react with certain components of the dispersion and aggregate to form a gel or sol of high viscosity, which may cause an undesirable increase in the viscosity or otherwise affect the rheological properties of the coating color.
Inventive efforts have therefore focused on the preparation of stable dispersions of pigments to be incorporated into the coating color in a manner that allows maximum compatibility with other components of the coating formulation. Such dispersions have been prepared using an acid-based dispersant as a means of stabilizing the pigment particles in suspension. For example, U.S. Pat. No. 3,935,023 (Derolf) discloses aqueous dispersions of alumina in a hydrochloric acid-containing solution. The dispersions so formed have an alumina content of from about 18-26% weight, based on the total weight of solids, or about 54-66% parts by weight (pbw) of Al2O3 per pbw of hydrochloric acid, and a specific surface area of pigment, as determined by the Brunauer, Emmett and Teller (BET) method, of 150-250 m2/gram. The patent points out that in order to achieve dispersions of 18-26% by weight, it is necessary to maintain an Al2O3 to HCl ratio of 54-65 pbw to 1 pbw of HCl, and that when less than 53 pbw of Al2O3 per pbw of HCl is used, the shelf life of the resulting product is drastically limited. Accordingly, the acid is a necessary component of such formulations. Acids, however, tend to be hazardous and corrosive materials, and are less effective towards achieving a dispersant effect. Moreover, it has been determined experimentally that using acids as the primary dispersion means will not produce slurries having the desired high level of solids as well as an acceptable viscosity.
U.S. Pat. No. 5,518,660 (Wei) describes preparing low-viscosity colloidal dispersions of sub-micrometer alumina particles for use in ceramics manufacture that contain 0.1 to 5% by weight of a semicarbazide hydrochloride acidic compound as a dispersing agent. In this regard, the semicarbazide compound dissociates in water by releasing the HCl ligand, which, in turn acidizes the dispersion to avoid agglomeration and thereby maintain a low viscosity. However, as noted previously, the use of acidic compounds in the dispersion is accompanied by the drawbacks of corrosivity and hazardous nature of such materials.
As an alternative, coating colors have been prepared by incorporating the solid pigment directly into the color formulation. For example, Japanese Patent Abstract No. JP 9314985A2 describes an aqueous coating formulation for inkjet or electrophotographic printing containing alumina hydrate, ethylenediamine-tetraacetic acid sodium as a chelating agent, an imidazolium compound as a cationic component and polyacrylamide. The process described by the patent abstract eliminates the step of dispersing the alumina hydrate and adding it to the color formulation as a slurry. Direct introduction of a solid pigment during preparation of the color formulation as reported in this abstract is undesirable, however, because additional steps are then required to homogenize the coating formulation and to maintain the desired rheological properties. These additional steps would increase manufacturing costs. In addition, the coating formulation may still be susceptible to the deterioration observed with respect to many dispersions, such as flocculation and gellation.
Accordingly, a need exists in the art for a method of preparing alumina pigment slurries having high pigment solids content, and which may be stably maintained over long storage periods. In addition, it is desirable that such slurries be highly compatible with ingredients used in coating formulations for paper and paperboard manufacture, while also being particularly useful in coatings for printing applications such as inkjet or electrographic printing.
A solution to the problems evident in the prior art is found in the present invention, which relates to stable pigment slurries that are prepared without an acid-based dispersant, and methods for using such slurries in paper or paperboard manufacturing applications. The slurries may advantageously be used in the manufacture of ink-receptive coatings for printing applications including, but not limited to the manufacture of inkjet paper coatings.
According to the present invention, a pigment slurry that is suitable for use in manufacturing paper and paperboard coating formulations may be prepared and maintained over relatively long periods without flocculation, gellation or other deterioration. The slurries of the invention may be used in a variety of applications including, but not limited to, paper and paperboard coating color formulations. The slurries are formed by dispersing and homogenizing an alumina pigment, a nonionic wetting agent or nonionic polymer and a cationic interfacial modifier in an aqueous medium.
As used herein, the term xe2x80x9calumina pigmentxe2x80x9d includes particulate aluminum oxides and hydrated aluminum oxides in activated, calcined or fused form. Preferably, the alumina pigment may be selected from the group consisting of fumed alumina, alumina trihydroxide and pseudoboehmite. One or more of the aforementioned compounds may be included as the alumina pigment component in the present invention. A wide range of pigment particle sizes may be used, the chosen particle size being limited only in that the particles should be small enough to facilitate suspension or dispersion in an aqueous medium. In this regard, larger particulate aluminas may, if necessary, be divided, for example by jet-milling, to produce a mean particle size suited to aqueous dispersion. A typical particle size for this purpose is in the range of from about 1 to about 10 microns, preferably from about 0.05 microns to about 3 microns. An example of such a material is a pseudoboehmite manufactured by Alcoa World Chemicals Inc., and subsequently jet-milled to a particle size of about 2.0 microns. As another example, a fumed alumina having an average particle size of about 0.2 micron may be used. While it is desirable that a pigment of a particle size sufficient to permit suspension in the aqueous medium of the invention be used, the physical characteristics of the pigment are not otherwise limited. For example, an alumina pigment having a mean particle diameter of about 2 microns was used to prepare slurries according to the invention, the BET surface area, which is a measure of how finely the pigment is divided, of such a pigment may be as high as about 300 m2/g, typically about 270 m2/g, while its pore volume may be in the range of up to about 0.5 ml/g. The alumina pigment preferably represents from about 35 parts by weight to about 45 parts by weight of the aqueous slurry, based on the total weight.
The nonionic wetting agent used in the invention is a non-particulate additive that reduces the length of time otherwise required for wetting out of the alumina pigment in the aqueous medium by eliminating the interfacial separation between the pigment particles and the water molecules in the slurry. Typically, this agent is of a molecular composition including at least one alcohol functionality, and, accordingly, may be selected from mono- or polyhydric alcohols, polyalcohols and polyols. Preferably, the nonionic wetting agent is an acetylenic alcohol such as 3,5-dimethyl-1-hexyne-3-diol, which is commercially available for example, from Air Products and Chemicals, Inc. A nonionic polymer may be included as an alternative to or in combination with the nonionic wetting agent. Such polymers are polar in nature, and function as steric blockers that prevent agglomeration of the pigment particles. Examples of appropriate nonionic polymers include nonionic polyacrylamides, such as polyvinyl pyrrolidone (PVP), and nonionic polyvinyl alcohol (PVA) polymers. The amount of nonionic wetting agent, nonionic polymer or combination thereof to be included in the slurry is from about 0.5% to about 5.0% by weight, preferably from about 2% to about 4% by weight, based on the total weight of the pigment.
The slurries of the invention also include a cationic interfacial modifier. As used herein, the term xe2x80x9ccationic interfacial modifierxe2x80x9d means a cationic dispersant, which acts as an antinucleation agent, or a cationic surfactant, which lowers the interfacial tension between the pigment particles and the molecules in the dispersing medium, or any combination thereof. The cationic interfacial modifier includes, but is not limited to, oligomeric compounds, such as inorganic oligomers, and other organic or inorganic polymeric compounds. While not wishing to be bound by any particular theory of operation, it is postulated that the cationic dispersant or cationic surfactant facilitates stable suspension of the pigment particles in the slurry of the invention by establishing a delocalized positive charge throughout the aqueous medium. The pigment particles are thus electrostatically attracted to and separated by the cationically charged species and, as a result, do not aggregate to form larger particles that flocculate from the medium, or form gels of a viscosity so high as to reduce the spreading ability of any coating containing the pigment. This delocalized cationic charge throughout the dispersing medium, in combination with the functional effects of the other ingredients, such as the wetting effect of the nonionic wetting agent or the steric blocking effect of the nonionic polymer therefore facilitates suspension of the particles in the slurry, and may be responsible for the prolonged stability of the slurry over storage periods of up to one year or more.
Preferably, the cationic dispersant is an inorganic oligomer. One example of such a compound is an aluminum hydroxychloride oligomer sold under the trade name xe2x80x9cSYLOJET 200Axe2x80x9d, which is commercially available from W. R. Grace Inc. The cationic surfactant may be used instead of or in combination with a cationic dispersant. A suitable example of this component is RHODAQUAT T, which is a quaternary ammonium compound having a dispersant and antistatic effect, which is commercially available from Rhone-Poulenc Inc. The amount of eitherctionic dispersant or cationic surfactant that may be included in the slurry is from about 2% to a bout 15% by weight, preferably from about 5% to about 10% by weight of the total weight of the slurry. In various embodiments of the invention, a combination of cationic dispersant or cationic surfactant may be included in the slurries. In this regard, the proportions of each of the polymer dispersant or surfactant are selected so as to provide a total amount of cationic interfacial modifier of from about 2% to about 15% by weight, preferably from about 5% to about 10% by weight, as described above.
When the alumina pigment is a fumed alumina, a supplemental dispersant such as lactic acid may optionally be added to the slurry in an amount sufficient to generate a pH of from about 2 to about 6. This reduction in pH is generally accompanied by a reduction in viscosity. This ingredient is not necessary for forming the slurries, however, in the case of fumed alumina, mixing with a cationic dispersant such as aluminum hydroxychloride oligomer as well as lactic acid enhances the dispersion of the cationically charged pigment particles in the suspension. This combination augments the dispersion of the fumed alumina to make a stable slurry having an advantageously high solids content. In this embodiment, a fumed alumina slurry with a solids content of up to about 42% weight may be obtained.
To make the slurries, the alumina pigment, nonionic wetting agent or polymer and the cationic interfacial modifier are combined in water, using any conventional mixing means, and agitated to form a slurry. While the order of combination of these ingredients is not critical, it may be desirable to first combine the pigment with the nonionic wetting agent to more easily disperse it in the aqueous carrier before adding the remaining ingredients. After mixing, the resulting slurries have been observed to have an alumina pigment concentration of up to about 67% by weight of solids. Preferably, the pigment content is in the range of from about 42% to about 67% by weight. The viscosity of the resulting slurry is usually on the order of less than about 5000 cPs, preferably from about 500 to about 2000 cPs, as determined at 20 rpm using a Brookfield No. 4 spindle at 25xc2x0 C.
Other additives typically used in paper and paperboard pigment formulations may be added to the pigment slurries of the invention. Such additives include, without limitation, one or more ingredients selected from the group consisting of other pigments, brighteners, defoamers, binders and other conventional slurry additives.
The slurries may be implemented in a wide range of applications where pigments are used. They are particularly suitable for forming pigment-rich coating color formulations for paper and paperboard coating applications when combined with other coating components. The amount of slurry used in such coating color formulations is that which is effective to provide the maximum amount of alumina pigment in the final coated product. When the slurry is used in a coating color formulation, the color may be applied to at least one surface of a paper or paperboard stock by any conventional means known in the art, for example using a roll, blade, bar or pad coater, and the excess metered off to form a layer of uniform thickness on the surface of the stock. Other coating methods may also be used. For example, the coating color may be applied as a metered film or by cast coating. After coating, the layer of coating color may then be dried, and the coated stock finished by any conventional means, such as calendering. As but one example of their application in paper manufacture, the slurries may be used in conjunction with a combination of porous and non-porous pigments that yield excellent print performance on original equipment manufacturers"" (OEM) inkjet printers. In this regard, it has been observed that the performance of coatings made with the cationic pigment slurries of the invention demonstrate superior printing performance and hold out of anionic inkjet inks.
The slurries formed according to the invention are particularly compatible with other strongly cationic materials that may be used in coating formulations. Non-limiting examples of such materials include cationic latex emulsions and Poly(DADMAC), which is a cationic quaternary amine.
Moreover, the cationic pigment slurries of the invention are highly stable. As used herein, the term xe2x80x9cstablexe2x80x9d means that the slurries may be stored after preparation without subsequent mixing or agitation for relatively long periods of up to a year or more, without the occurrence of gelling, flocculation or other undesirable deterioration is eliminated or significantly reduced. This stability may be attributed, in part, to the fact that an acid-based dispersant is not used, and also to the stabilizing effects of the cationic interfacial modifier and the nonionic wetting agent and/or polymer.